The present invention relates to optical recording apparatuses for recording pulse signals on optical record mediums.
FIG. 7(A) shows pits recorded on an optical disk and an output RF (high frequency) signal waveform produced by a pickup means (not shown) when the pits are read. The length and distance of each pit respectively correspond to data lengths of signal portions "1" and "0" of a recorded digital signal which is not shown in this figure. For example, pit 1T corresponds to unit bit length "1" of the digital data signal and pit 3T corresponds to bit length "111" of the digital data signal and the length of pit 3T is correspondingly three times that of pit 1T.
A synchronization signal is separated from the above RF signal and is supplied to an unillustrated PLL (phase locked loop) circuit, thereby providing a data-demodulating clock signal which is synchronized with the above synchronization signal and follows the change in time axis of the above RF signal. Through use of the clock signal, the signal is sample-held and A/D converted and demodulated to a bi-value signal indicative of value "0" or "1". This sampling timing and the respective values are shown by the arrows in FIG. 7(A).
The length of the pit recorded on a CAV (constant angular velocity) disk is relatively long on the outer circumferential side of the disk, and the distance between the respective pits is also relatively long. In such a region, the amplitude of the RF signal outputted from the pickup sufficiently exceeds a predetermined judging level even in a signal portion having a unit pit length. FIG. 7(A) may be considered as representing pits recorded on the outer areas of a CAV disk.
However, for digital signals recorded on the inner circumferential portion of an optical disk, as shown in FIG. 7(B), the length of each pit and the distance between the respective pits is relatively narrow. When the pit length is shorter than a certain length the contrast in reflected light of a light beam returned from the pit in an optical pickup system is lowered. Thus, as shown in FIG. 7(b), since the output of the pickup is reduced, the signal portion corresponding to a pit having a unit pit length has an insufficiently low voltage level.
Accordingly, the difference in voltage level between adjacent signal portions corresponding to level "0" is reduced so that the so-called jitter margin with respect to the change in time axis of the clock signal is reduced.
A prior circuit as shown in FIG. 8, for adjusting the width of a pulse prior to recording on an optical disk is used to increase the above jitter margin.
In FIG. 8, input digital signal P having a waveform as shown in FIG. 9 is delayed by signal delay circuit 41 by predetermined time t to obtain delay signal Q. Delay time t is set such that this delay time does not exceed a time corresponding to the unit bit length (minimum time length) so as not to change the information content of the signal. A logic product operation between input digital signal P and delay signal Q is performed by AND gate 42 to obtain output signal R having a pulse width which is narrower by time t than the pulse width of input digital signal P. Further, a logic sum operation between input signal P and delay signal Q is performed by OR gate 43 to obtain output signal S having a pulse width which is wider by time t than the pulse width of input digital signal P.
Pickup position detecting circuit 45 is constructed by e.g., a potentiometer, a position detecting switch, and other conventional means, and detects the radial position of the pickup relative to the disk. When pickup position detecting circuit 45 detects that the pickup is located on the outer circumferential portion of the disk, pickup position detecting circuit 45 causes signal selecting switch 44 to transmit output signal R to an output terminal thereof. When pickup position detecting circuit 45 detects that the pickup is located on the inner circumferential portion of the disk, pickup position detecting circuit 45 causes signal selecting circuit 44 to transmit output signal S to the output terminal thereof.
Thus, when the pickup is located adjacent the inner circumferential portion of the disk, the pulse width of the record digital signal is widened and the length of the unit pit can be longer by the above-mentioned pulse width adjusting circuit.
However, the above-mentioned pulse width adjusting circuit increases not only the pulse width of the minimum bit portion of the digital signal but also the pulse width of the other bit portions of the digital signal so that an unrecorded area between the pits recorded on the disk becomes relatively narrow and the interference between codes is increased as a whole.